Solid core golf balls are well known in the art. Typically, the core is made from polybutadiene rubber material that provides the primary source of resiliency for the golf ball. A known drawback of polybutadiene cores cross-linked with peroxide and/or zinc diacrylate is the adverse effect of the absorption of water vapor, which can degrade the performance of the core material, in particular resilience.
Prolonged exposure to water vapor (i.e., at high humidity) and elevated temperature may be sufficient to allow the water vapor to invade polybutadiene cores, reducing the initial velocity of the golf ball by about 1.8–4.0 ft/s, sometimes greater. The change in compression may be reduced by 5% or more. Absorbed water vapor can also reduce golf ball coefficient of restitution (“COR”). When a golf ball is subjected to prolonged storage and/or use under humid conditions (i.e., 25–35% relative humidity), or conditions exhibiting a combination of high temperature and humidity, the COR of the golf ball decreases over time due to water vapor absorption.
Therefore, cores of this nature must be covered quickly to maintain optimum ball properties. A cover that protects the core from the elements and repeated impacts from golf clubs is typically made from ionomer resins, balata, or urethane, among other materials. Ionomer covers, particularly hard ionomers, offer some protection against the absorption of water vapor. However, it is more difficult to control or impart spin to balls with these types of covers. Urethane covers can provide better ball control but offer less resistance to water vapor than do ionomer covers. Several prior patents have addressed the water vapor absorption problem. These include U.S. Pat. Nos. 5,820,488; 5,885,172; 6,132,324; 6,232,389; and 6,287,216, which are incorporated by reference herein, in their entirety. Manufacturers have typically resorted to the incorporation of a WVB material in an existing layer or a designated WVB layer to overcome the water vapor absorption problem in golf balls, escalating manufacturing cost and construction complexity. It is therefore desirable to provide cost-effective material compositions that have general characteristics suitable for golf ball constructions with concomitant WVB properties.
One particular class of polymers suitable for golf ball compositions of the present invention is fluoropolymers. Fluoropolymers are well known for their excellent resistance to outdoor weathering and ultraviolet radiation, high degree of physical toughness, chemical inertness, water and gas impermeability, low moisture absorption, good abrasion resistance, non-tackiness, resistance to soiling, as well as a significant retention of these properties at both low and elevated temperatures. This rare combination of properties is due to the strength of the C—F bond. Other desirable properties of the fluoropolymers include high elongation, low coefficient of friction, low wear rate, good resistance to swell in solvents, low refractive index, and low dielectric constant. These desirable properties allow the fluoropolymers to be widely used as self-supporting layers or outer coatings on various substrates. Disclosures of fluoropolymer applications in golf ball compositions, however, are relatively limited. These include U.S. Pat. Nos. 5,962,140; 6,133,389; and 6,217,464, which are incorporated by reference herein, in their entirety.
On their own, fluoropolymers as inner golf ball layers generally have poor impact durability. Attempts to impact modify fluoropolymers by blending with functionalized polymers, such as grafted polyolefins, grafted block SEBS polymers (i.e., Kraton®), low modulus ionomers (i.e., VLMI), and highly-neutralized polymers, have been largely unsuccessful. There remains a need, therefore, for fluoropolymers modified to improve impact resistance of golf ball inner layers formed form such materials.
It has been determined, however, that the use of alkyl acrylate based block copolymers, such as methyl-methacrylate-styrene-butadiene (“MMA-SBS”), or hydrogenated derivatives thereof, impact modify non-ionic fluoro-copolymers (“IM-NIFP”) in a manner such that the impact resistance of golf ball layers formed of these materials is greatly improved. It has also been determined that the materials the present invention provide added benefits, above and beyond the impact modification including, but not limited to, improved moisture barrier properties, high specific gravity, and increased flexibility with respect to tailoring material properties (by varying the ratio of fluoropolymer to alkyl acrylate block copolymers and/or varying the block components themselves (i.e., low to high methyl methacrylate levels)).